1. Field of the Invention
The field of this invention resides in the area of manhole frames and more particularly relates to adjustable manhole frames.
2. History of the Prior Art
The standard practice in manhole installations is to supply pre-cast concrete manhole sections to be assembled on the job site. These manholes extend from the depth of the pipe to 12 inches below the road surface. This 12 inch space is for the installation of the frame and manhole cover which vary in height from 4 to 10 inches. The void between the bottom of the frame flange and the concrete manhole is filled with either brick or mortar, cement block and mortar, or pre-cast concrete rings mortared in.
After installation of the frame, available fill material consisting of gravel or a combination of gravel and dirt is deposited around the exterior of the manhole and frame, and the fill is then compacted with whatever means is available, leaving 2 inches or more on top for the final paving material. Invariably, settling occurs within a short period of time resulting in a separation and cracking of the final paving around the frame. This settling process develops potholes that require constant patching, and over a period of time seepage of surface liquids such as water, gasoline, oil, and in cold climate areas, water carrying salt causes deterioration of the support base under the frame. In areas of severe deep frost penetration, entire sections of frame and base are lifted above the road surface. A great number of structures have to be repaired which means digging out and rebuilding the bases.
Where all brick and mortar manholes exist in the older systems traffic shock along with freeze and thaw cycles loosen bricks in the cone section as deep as 2 feet or more below the road surface. This situation usually requires extensive excavation and rebuilding of the entire cone section.
At the present the vast majority of manhole frame members are of a non-adjustable type, therefore any needed change means digging them out to raise, lower, repair or replace worn or broken frames.
The horizontal stability of prior art frames having a flat base surface resting on the manhole cone or brick support depends upon the surrounding backfill and pavement to hold the frames in position. Frequently, especially during wet periods, such frames can be forced laterally out of position over the manhole which displacement helps initiate the deterioration of the frame support and ultimate collapse of the frame.
It has long been desirable to have a simple and easy way of providing an adjustable manhole frame member to avoid the necessity of digging up the entire manhole frame out of the ground which project entails a great deal of labor with air compressor equipment and jack hammers to cut through the pavement and to loosen the compacted fill with the accompanying great expense and traffic disruption. In addition, the base support may also have to be dug out and rebuilt due to instability of either or both frame and support base as inevitably traffic moves the frame while freeze and thaw cycles move and destroy the base.
In an attempt to raise manholes without the necessity of digging out the entire structure, a prior technology utilized ring members for placement upon existing frame cover seats into which the covers are replaced. These ring members have limited use as their minimum overall height is greater than the thinner layer of pavement used today. Also, if they are not carefully installed or do not properly fit the cover seat and cavity, they could be dislodged and cause traffic accidents.
Many attempts to create adjustable manhole frames have been made. One is the "Preko" adjustable manhole frame disclosed in U.S. Pat. No. 3,858,998 which includes a second frame within an outer frame with screw adjustment means to raise or lower the inner second frame member in relation to the outer frame member so that if the road is raised, one turns the screws on the manhole, and the inner frame will raise or lower the manhole cover to the desired height. These "Preko" adjustable manhole frames have not gained widespread usage in the industry because of their higher initial cost due to the complexity of machining that must be done to create them and to the inherent fear that the screw elements might jam and fail to operate after the frame had been installed for a long period of time.
Another adjustable type is the "self-level", a United Kingdom invention disclosed in U.S. Pat. No. 4,174,183. This frame is installed during paving operations which requires hot pavement and also the use of a pavement roller for compression of the paving material and the positioning of the frame. A special exterior concrete ring is also required which is mortared to the base manhole. This frame is limited to shallow height adjustments, and when the frame is raised, a void can be created that could trap water which, if frozen, could force the unit up into the traveled way. Further, unless care is taken to properly fill and compact around the frame exterior, surface and underground liquids could undermine and/or deteriorate the frame support.